Factual error: Hernando Cortes is featured as the main background villain. Only thing about his appearance: according to the history books, he never actually ventured into South America to do any conquests; his actions were limited to Cuba, Mexico and California.
Factual error: The real El Dorado (Spanish for "the gilded one") civilization was in South America. Specifically, El Dorado is a legend coming from the Muisca people of Colombia. The art scheme of the film is Mayan, a Central American civilization, as are the names of the Indian characters. The ball game, too, although not restricted to the Mayas, is, culturally speaking, Central American (and more specifically, and technically, Meso-American). In addition, the Mayan architectural elements found in the movie are typical of the Mayan culture of the Classic Period (c. 250-900 C.E.), roughly 600 years before the arrival of the Spaniards.
Answer: Miguel had just told Chief Tanni how loyal Tulio and he were to each other. Then he overheard Tulio saying to Chel that he wanted to go to Spain with her, ONLY with her "Forget Miguel." That's why Miguel was so upset, because Tulio excluded him from his plans and his life.